In many wireless communications systems, such as Universal Mobile Telephony Systems (UMTS) and other third generation (3GPP) wireless communications systems, a communications device is often put into a sleep mode in order to decrease power consumption and increase battery life. The communications device can then wake up at fixed intervals to check if it is the recipient of an incoming page. In UMTS, this is referred to as the DRX mode, or discontinuous reception mode.
To further reduce the amount of time that the communications device needs to be active when it is operating in the DRX mode, the communications device can decode an indicator channel which can contain a Boolean value specifying if it is the recipient of an incoming page. If there is no incoming page, then the communications device can go back to sleep. If there is an incoming page, then the communications device can decode the paging channel to receive the details of the incoming page (or simply, receive the incoming message).
The quality of the wireless channel can have a significant impact on the accuracy of the decoding of the channel. For a low quality channel, perhaps due to the communications device being far removed from the base station to which it is communicating or the communications device being operated within a tunnel or large building, the signal quality of the channel (commonly referred to as signal-to-noise ratio (SNR)) can be low. When the SNR of the indicator channel is low, then the probability of the communications device erroneously decoding the indicator channel can be high. Conversely, when the SNR of the indicator channel is high, then the probability of erroneously decoding the indicator channel can be low. If the indicator channel is erroneously decoded, the communications device may erroneously decode the indicator channel one of two ways: a false alarm or a missed page. With a false alarm, the communications device would decode the indicator channel as indicating that there is an incoming page when there isn't one, thereby unnecessarily increasing power consumption and reducing battery life. While with the missed page, the communications device would decode the indicator channel as not indicating that there is an incoming page when there is an incoming page resulting in a missed call (since the presence of a page is usually followed by a call).
A commonly used method for detecting the state of the indicator channel is referred to as the maximum likelihood (ML) method. The ML method uses a zero threshold and assumes that the a priori probabilities of being paged and not being paged are the same and that the costs of erroneously decoding the indicator channel (the false alarm and missed page) are the same.
Another commonly used method for detecting the state of the indicator channel is referred to as the Neyman Pearson (NP) method. The NP method permits a threshold to be set with unequal false alarm and missed detection probabilities. The NP method is typically used to set a performance metric, such as a constant missed detection (or false alarm) probability across the range of SNRs, and then the other performance metric, i.e., false alarm (or missed detection) probability can be automatically determined. Essentially, there is one degree of freedom since there is only one threshold to set with two performance metrics. Therefore, setting the threshold based upon one performance metric would result in the automatic determination of the other performance metric.
One disadvantage of the prior art is that the ML method permits only the assignment of equal costs for erroneously decoding the state of the indicator channel as either a missed page or false alarm. Furthermore, it also assumes that the probability of being paged and not being paged are equal. These assumptions are incorrect in real-world applications and their use can result in poor performance.
A second disadvantage of the prior art is that the NP approach does not adapt the threshold by taking into consideration the fact that the sensitivity of the overall performance metrics (battery life and missed call rate) to PICH detection may change over an entire range of SNRs (or equivalently, the location of the communications device in relation to a cell site). Rather, the NP approach attempts to keep either the missed PICH rate or the false alarm probability constant over the entire range of SNRs.